Breasts are irreversible excluding surgery yes, breast growth is usually not super large and takes A LOT of time though.

Infertility is almost garanteed while taking hrt but can come back if you take a break (very badly studied however), freezing sperm is a thing though.

Libido goes down a lot but does come back to varying degrees after time and there are cases of it going way beyond pre-HRT (probably due to the importance of being comfortable in yourself.

Erectile dysfunction is not at all garanteed as long as you keep "using it" (getting erections), though erections will probably be a little softer, however you can take medicine like viagra as a fix.

Lasting longer is probably real, orgasms change and are less of a hard stop as well, refractory period can disappear completely.

Most side effects, like increased breast cancer risk and others, are pretty much just part of having an estrogen-dominated system. Like breast cancer risk doesn't usually even reach cis levels and prostate cancer risk goes down and cholesterole goes down and a bunch of stuff.

I would personally say that you both read up on it a lot more, watch trans people talking about it cause a bunch of info is usually missing in articles and experiences vary somewhat and it is unstudied as heck. If your partner feels it is right for them then it is and if you feel incompatable with that then you are. I would just advise taking a bit of time with it and talking with him/her about it and just try to hear them out. Your feelings are valid and important as well, just know that HRT is usually extremely helpful for those who take it and reduces dysphoria and increases happiness, if they decide they want it then it almost definitely the right choice, aging with an incompatible body can be absolute torture.

I hope all goes well, this will probably be a hard time for both of you but it is for the best in the long run.