Frankly, an expert comptable would be massive overkill for your purposes. For something like 40€ you can get ClickImpot or another computer based tax declaration program. As long as your husband still reads French reasonably well, he should be able to figure it out - the instructions for the foreign source income form have been rewritten in recent years to be pretty clear, especially on the matter of pensions.
If you're thinking of going the AE route, just be advised that as part of the "simplification" of the system, you are not obliged to keep financial records other than recording all turnover. If you were to engage an expert comptable, you get no deduction nor credit for that expense in your business. Oh, and also, just a note - an AE is a one person operation. You each need your own AE registration (and need to keep separate track of your respective revenues).
Cheers,
Bev