I have two best girlfriends with whom I do this. Both are significantly heavier than me. One is highly motivated and keeps pace with me completely and even surpasses me, which honestly blows my mind.
She's amazing! The other needs a lot of hand holding, which is fine.
Who knows which way your friend may go, but she's going to need very different things at the beginning. With my friend, I just started walking with her to help keep her motivated. Walking with her is not a good work out for me. It's not long enough or fast enough to benefit me much. So I look at it as just more movement in my day, and there's nothing wrong with that!! But it's not my "workout". But I would never tell her that! I tell her things like, "Wow, we walked the same distance in less time today," and stuff like that.
I just try to take her where she is and I try to let her take the lead on weight loss talk. It's hard because I like to talk about it! I'm sure I'm not perfect at it and I probably say stupid things, but I try.
Mostly I'm there as a listener and a motivator and I share what I know WHEN ASKED. She's got to find what works for her on her own terms, which is hard for me. I'm a "fixer" by nature.