Is taking a postdoc for training purposes wise as an industry scientist?
Sorry, this doesn't seem to be a good idea, especially considering some of the more subtle points you've shared. In general taking post-docs after 5-6 years need not be a bad thing, but people would generally shoot for a prestigious fellowship, not something through personal contacts.
I also want to recommend that you take this just because of how unhappy the current role makes you, but considering your situation I can't. You're in that tricky (not uncommon) position where your PhD, current role and future aspirations are in different fields. As such, you should be very wary about letting go of what you have for something that you potentially may get. There is every chance that the grass appears greener on the mass spectrometry side, and after being trained on it (by a grad student!), you may find that there are difficulties and unpleasant challenges that you aren't currently aware of.
You seem to move towards mass spectrometry in the long term, and I get the uneasy feeling that you're jumping onto the bandwagon without enough thought. Primarily because if you really want to make a big switch, getting a nameless (i.e. without a fellowship) postdoc and being trained by a grad student (and neither earning certification nor being under the wing of an experienced academic) doesn't sound wise.
There's every possibility that I'm assuming too much, so please use your discretion and don't be hasty.