I still would suggest starting very gently. I'd start by trying to simply flush it with a little sqirt of two, then use a small tissue or paper towels (maybe a cue tip).

If that failed, then mayb carefull tap it. But you're going to be pushing "corroded" stuff into the non-corroded steel. When you're done, it's not the same as tapping good metal.

Anyway, that's my view. The other option will cost a lot more money if it doesn't work. He'll need a new pump body assembly. The part will be expensive...