I preface this by saying I am biased, and here is why:
My cinematographer worked for years for no pay on multiple projects with a very skilled director/gaffer. My cinematographer never had projects of his own, so he just worked on this director’s projects pro bono. Flash forward about four years, and my cinematographer finally has a project he’s working on with me, and we need a gaffer. Our project is very low budget, short on crew, short on time, and my first time directing a short. This director/gaffer steps in literally 12 hours before call time, brings a ton of gear plus some craft services, and elevated our project enormously. He saved our butts, and took our work to the next level. My cinematographer and I are so grateful, and we’re going to help this director/gaffer in the future because we feel very indebted to him.
TL;DR: Someone we’d work with for free paid us back big time, in time and skill. In return, we’re willing to help him big time in the future.
Trading work is based on a lot of trust. Would we hate this director if he had told us no? Probably not, but we wouldn’t be willing to help him for free again. I think unless you’re willing to risk losing the relationship (you probably won’t get this individual to work for you for free again), you should help them at least a little. Be really honest about your time (“I can only devote X hours to this each week/in total”) and then stick to your own boundaries.
Then again, maybe this person wasn’t vital to your sets in the past, and you’re okay with losing the relationship! No shame in that.
Sorry for the long answer, hope it helps!