to answer everything you've asked:
how ghost would be to a long time friend who helped him during his struggles since childhood?
ghost seems like a big introvert, even before the military. now i really do think ghost was that one awkward, quiet guy in the back of the classroom who had little to no friends. considering the fact he came from an abusive household, he was probably known as the kid who had a druggie dad so no one really interacted with him :( i genuinely headcanon him to be really close to his brother tommy as a kid, because they truly only had each other. i don't think hes completely anti social and afraid of having friends back, but he definitely was not a stereotypical extrovert.
ghost would probably be very open and kind to that friend helping him throughout his life, the one person who didn't think he was weird. in fact id arguably say ghost probably had a small crush on them when he was a kid, since no one really stuck around with him.
how he would act around them?
before joining the military, he would probably just act like any teenager around them. sharing jokes, listening to music together, going biking.. whatever teenagers in the 2000s did.
however, after the death of his family and joining tf141, he would likely ghost them.
what he would do or react to them joining the military as him, especially if they seen each other for the first time after years
if that friend joins the military and he sees them, he would probably awkwardly shoot up a 'hi' and then 'bye'. he doesn't know how to approach someone after that long.
does ghost try to stay in contact with said person?
i dont think he would. primarily for safety reasons. maybe he would stalk them on social media (if the account is public) and reminiscent about the old days, but other than that, i dont think he would actively try to contact the person.
if that person contacts him however, he won't decline the offer to meet up.
does ghost think his longtime friend/possible bestfriend also thinks he murdered his family and is dead or do they know ghost long enough to know he wouldn't do that type of thing?
depends on the person of course. but i feel like that longtime friend has trouble believing it, but if it was confirmed on the news, they probably believe it to some extent. after all, knowing ghost was an abused kid + a troubled SAS soldier it would be logical for anyone to think he did it. ghost would think the person thinks hes a murderer. but i dont think that longtime would be scared if they saw ghost again, more like surprised and willing to connect again.
does ghost think they forgot about him or did ghost forget about them after years?
the thing about ghost is that he doesn't forget. never. he has sharp memory and values the people he cares about very deeply. he would never forget. ghost probably thinks they forgot about him though, after all, its been decades and that longtime friend probably already has a family.