I hope you don't mind me hopping in on this! Full disclosure, I'm not a medical professional, so all of the following is based on personal experience and research, anecdotal experience of people around me, and some admittedly surface-level research around topical T.
Tl;dr: topical T may not be the best solution, but it might work for what they're hoping for. There are plenty of other options, through toys, medical devices, and/or dietary supplements that could achieve their desires results more directly.
So when you say "small differences...to their genitals," I'm assuming you're referring to changes in shape and/or reduced size due to tissue atrophy (both erectile tissue and the supporting muscles). Topical testosterone can help with that, but it's definitely not the only option! When I was first transitioning*, it was pretty common advice to tell transfems that we have to "use it or lose it," i.e. you have to regularly get and maintain erections if you want to minimize atrophy. Feminizing HRT doesn't directly cause atrophy, but it does drastically reduce a kind of "maintenance" erection, spontaneous erections throughout the day and especially overnight that exercise those tissues to prevent atrophy. So the most direct way to combat it is to give yourself (or your partner) regular erections through either 1) stimulation or 2) healthcare products.
1) Stimulation, obviously, can be a complicated issue if someone is dealing with genital dysphoria, but it doesn't have to look like traditional "jacking off" or whatever. Difference kinds of manual stimulation can achieve the same effect, as well as toys. Any vibrator (but particularly rumbly ones like wands) can do it, or more penis-oriented toys like strokers. For transfems in particular who want strokers, I tend to recommend ones that do suction or ones that don't require you to be hard the whole time**.
2) Penis pumps are great! I know that a lot of people think that they're scammy products about making your penis bigger, but the truth is that they're primarily medical devices used to treat erectile dysfunction. I spent several years in adult retail, and most of the guys buying pumps got sent to us by their doctors to treat their ED. This one in particular is super safe*** and is the only (US)FDA regulated pump on the market, plus they have heavily vetted prescribed exercise regimens you can follow, regardless of whether you're using their product or not
Now, on to the other part of your question: ejaculate volume. This one is a near-universal experience of folks on feminizing HRT; some of us love it, some of us hate it, some of us couldn't care less. Topical testosterone won't directly impact ejaculate volume, but it can promote the health of one's prostate and seminal vesicles, leading to an increase in seminal fluid (a major component of ejaculate). Then again, it may not, this (like a lot of HRT) tends toward the "your mileage may vary" side of things, rather than a definite, or likely, result****.
There are, however, other options for increasing ejaculate volume! Personally, I had a genuinely fantastic experience with Popstar's Volume & Taste Enhancer. It took 3-4 weeks to fully take effect, but once it did, it drastically increased my volume of ejaculate*****.
If you don't want to go the "random sexual supplement" route, I found this post a couple of years ago where a trans girl DIY'ed various herbal and nutritional supplements, and ended up with a very similar blend to Popstar's. A friend of mine has recently started following the regimen described in the post, to similar effects, but for MUCH cheaper overall. Popstar is ~$50 a month, whereas her DIY method comes out to ~$9 a month. I think it did have a steeper buy-in cost initially, but she's getting larger orders of the supplements that balance out the price over time.
All in all, your partner has plenty of options to address any disappointments or issues she's had as a result of HRT, and if you or they or anyone else wants to reach out to discuss their experiences or anything, my DMs are always open!!
*It still might be, it's been a long time since I was regularly around early-transition communities
**Ignore the branding. No, seriously, it's awful. Even the guys that buy them hate the word "guybrator" and they just won't stop using it. Also the box makes the toy seem cheap and shitty, but I promise it's not.
***With the exception of their Hydroxtreme line, the absolute maximum pressure you can achieve with a Bathmate pump is a body-sale level of pressure.
****Admittedly, I don't know many transfems, and I don't know of any studies done on this topic specifically, so maybe it's more likely than I'm assuming.
*****Seriously, ORDERS of magnitude. Without it, I usually have a few drops that trickle out, occasionally I'll have maybe a tablespoon. At the peak of being on it, I literally couldn't get off without a towel under me, and a willingness to accidentally hit myself in the face.
