Really depends on the client and your alternatives. Being the guy that "solves problems, even if they are out of the usual domain" can be quite valuable, assumi

Really depends on the client and your alternatives. Being the guy that "solves problems, even if they are out of the usual domain" can be quite valuable, assuming the client worth sticking with. On the other hand, "that's not my core thing and I'm more than busy with work that fits into my profile better" is also answer a good client should respect. But at least giving it a quick try to see if its something obvious is probably worth it. The suggestion in the other comments to take it but (transparently) farm it out longer-term if needed is also a good one. "Not my usual thing, but I'll solve it and help you find someone to take care of it".