I was always under the impression the patron would get pissed, I also figured that we could probably beat the patron in a fight since we know have obtained the most powerful weapons, spells, and armies in existence. (We suspect the patron is tiamat). We think it would be slightly risky but the party would probably win without losses
So in terms of calling on wizards or diviners I’m on good terms with, there were plenty of them, but all of them (along with most of faerûn) died in a catastrophe caused by a former friend of the party. There is a library with important resources on magical threats and general magic though, which may still be standing among the ruins
While this catastrophe started and progressed, the party timeskipped 100 years or so. The warlock was out of the timeskip (or so we think), and is unlikely to have kept any copy of the pact over the years
A wish is probably the best way to go about it, but the pact may have included some sort of clause about de-aging, considering the warlock is human and should not have lived 100 more years and hardly age at all.
I’ll discuss the plans with the DM and see where that gets us, hopefully there are no unforeseen consequences to the characters!