Foil tape would only be a problem if you have open circuits on your flexwatt. If you do the issue is not foil tape but with the person whom should never should have been working with wiring in the first place. Flexwatt once wired and complete should be well insulated and there should be no exposed wires anywhere. Foil tape or any other tape will make no difference as long as it is rated to beyond the max possible flexwatt temps.

this is why I'd not suggest duct tape. It is typically rated to 130ºF max. I tested my flexwatt to max at 160-170º I would stick to tapes that are rated to over 200ºF just in case. Overheated burned snakes are horrible and tragic but a burned down house is worse.

I would suggest that visual inspection of flexwatt is critical, I do mine every cleaning. I untapped each section and check it very carefully once a month during the regular cleaning. (it also allows my to disinfect the rack body at the same time) I'd also suggest with flexwatt in particular a fail safe t-stat is an important safety addition to any set up.