I am running modified version of  Storm King's Thunder and I have a Tiefling rogue who wants to multi-class into a warlock.  She has had dealing with a devil in her backstory and is opposed to the Fiend pact. OK. So, Celestial Pact is the path she has chosen and I am perfectly fine letting her play as one.  My question is how bad would it be to have her Celestial Patron actually be a Devil disguised as a Solar?  Would this be outside the realm of DM-player trust?  Or should I honor her wishes and create a legit celestial patron for her? Is it even possible for a Devil to give celestial powers?  Any thoughts would be appreciated!

Here is the main thing.

If she gets the mechanics she's after? Good.

Is she getting the story she's after?... Great.

I don't know your player, but I know if she will think this is an interesting plot twist.

Depending on the group, it might be good to get her buy in.  It's her character and she should have ultimate veto.  That said, if it's a secret she knows and the other players don't... She might be all for it.