Ask her. Be honest that you want to be there for her but sometimes feel a bit out of your depth and don’t want to say the wrong thing. Let her know you love her and support her.
Some good advice above and it’s hard without knowing your friend but sometimes she might also not want to talk about any of it. Yes check in, ask how she is but also take her lead and let her know that you’re open to talk/ listen as much as she wants or to do something totally different. IVF can be relentless and sometimes you just want a break of all the heavy stuff and to be ‘normal’ for a bit. Do something fun, let your hair down and put a pause on all the fertility stuff for a bit - it can really take over.
Don’t suggest adoption. She’s knows that may be an option, it might be something she’s already wondered about, but if she’s still actively in treatment she’s not exploring that option right now.
If you get pregnant be normal with her (it can feel worse being handled with kid gloves) but be sensitive to the likelihood she’ll be happy for you but sad for herself and might find it hard or need some distance. It’s not personal. If you have a difficult pregnancy don’t hide it but she’s not the one to complain to about how hard it is etc. if you do have kids try to catch up without them sometimes if you can. In a group of mums the conversation will very often lean towards children or catch ups will be in child friendly places which can feel excluding.
Understand that she might find it difficult to plan things even if she wants to. It can be hard to look to far into the future when you don’t know what that future will look like. And timelines for treatment can really takeover and you’re on the clinics schedule rather than your own.
I’ve had my own experience with infertility but have friends who’ve had a different experiences - it can be a real spectrum depending on your diagnosis, how you feel about becoming a parent, the likeliness of that happening, your finances, your support network etc. It can be a minefield but being open, honest and empathetic is always going to be the right way to go. Maybe you might say the wrong thing unintentionally, but she’s your friend and knows you’re not perfect but will appreciate you making the effort.
I hope things work out for your friend and that she can find peace with whatever the future might look like.