OK, I won’t tell you to decline the job, but that’s exactly what I would do, unless the ring was made by the originator of invisible setting, Cartier.
Invisible set stones have grooved girdles. They must be cut to fit exactly for each seat. Cartier used to have a diamond cutter and the setter working together to create such a piece. The cheap knock-offs we commonly see are not constructed with the same degree of precision and so will not have the same degree of security.
The main problem with tightening those stones is not the difficulty of doing it, it’s the fact that from that moment on you own the piece. Every time there is a problem with it, it will be your responsibility. When a stone inevitably falls out, you will be expected to replace it. If you don’t have the ability to source precisely sized diamonds and then cut a groove in the girdles, you will have to send it to someone that can.
If you still don’t want to decline the job, my advice would be to take it in with the understanding that you will not be able to guarantee the security of the stones in the future, and then send it to someone that specializes in invisible setting to do the repair. In the long run it will very likely be much cheaper to pay someone that is properly equ[pped for such work than to do it yourself.