I'm more mixed. I came from a GOS-3a, to a GOS-6a.
Fingerprint is a problem (though this probably isn't down to Graphene, and according to beta-release notes, improvements should be coming with the Android13 release)
I'm unable to get mobile data at the moment, though this also is not limited to Graphene-powered 6a's.
My impression (which is anecdotal, just 1 data point) is that the 6a has some rough edges. If your friend wants to be patient while bugs are worked out, then it seems like it should be a great phone.
For what it's worth, my experience on the 3a was immaculate, I donate to the GrapheneOS project when I'm able to, and I'm committed to sticking it out while they do their thing. the 6a will catch up to the Graphene standard, IMO.