How can someone who has not experienced fatphobia help anyone struggling with body image and weight issues?
If this person doesn't have an eating disorder that could be triggered by it, point out things you like. Myself and others have found that one of the most healing things is to simply be complimented.
Some of my biggest insecurities are things I love about myself now because someone I trusted complimented those traits. You have to be very careful with compliments, though. Don't give backhanded compliments, and don't insist or argue with them if they try to deny it. Just tell them how it is, and don't sit in the moment too long or it may make them uncomfortable.
If someone is struggling with their body image due to an eating disorder, I would suggest to only make comments about their looks if they have expressly told you that they aren't triggered by that. Otherwise, compliment other things about them. Makeup, style, mannerisms, and such.
There will never be a perfect way to help someone with these issues. I think if you're close enough to someone, you can have a gentle conversation with them about it. But you have to respect boundaries when it comes to these topics, and be careful.