What to do?

My wife plays a pure blizzard sorc, and she wants to do Ubers. She has the opportunity to make Infinity for her merc, or can do Chains of Honor and buy a torch for herself. She already has mostly optimal gear, I think she's missing SoJ and some skillers. What should she do?

Blizz isn't going to gain as much from the Infinity with

Cold Mastery. My vote is finish the gear on the character. That last bit of gear will likely give the last bit of - res anyways. Remember, mobs can only go down to -100 res anyways.

A sunder charm changes this up, and I'm not sure about how effective Infinity is with

Cold Mastery, but my vote stands with other gear.

As for can a blizz sorc do ubers, I have no idea

If she doesn't have any torch, get one. Infinity isn't super helpful for cold sorcs, since they already have a huge -res from CM.
Regarding CoH, depends on the rest of her setup, where the best bang-for-your-buck improvements are.

Generally though, I think blizzard sorcs are gonna have a really, really hard time doig ubers.

You can do Ubers together, more fun and less of a very hard task for her

Alternate opinion: once you get Infinity the game changes, a whole bunch of other builds go from being meh to S tier. Torches are really good, one of the best items in the game, but they don’t change the game the way an Infinity will.

Infinity is also super hard to get, so if you have the opportunity to make one defiantly do that. It will help out a blizzard sorc too. There’s lots of builds that can do Ubers these days, torchs can be found cheap too.