I would be okay with it, both because of experience with same-sex couples having commitment ceremonies--and sometimes referring to themselves afterward as "married' and using the words wife and husband--and because I have a close friend in the same situation as your friend. She and her partner have been using "partner" and/or "fiance" for years, but are unlikely to get legally married unless the social security disability and medicare rules change significantly.
However, given what you say of your friend's plans, I'd suggest that she double-check that she isn't living somewhere that still recognizes common-law marriage. If she is, I suspect the combination of the ceremony with wedding-like vows in front of friends, and possibly referring to each other with the words "husband," "wife," or "spouse," might lead the government to decide that they are married, with the consequences she's trying to avoid, where "partner" might be safer. A commitment ceremony by itself probably wouldn't, but I am not a lawyer, and this is not legal advice, only a suggestion that they might want to at least do a bit of googling if they haven't already.