Probably not too good for the belt. It is common to use plain water. But I have used other stuff in a spray bottle that happened to be handy myself - but at least water based.

If it is a serpentine belt, does it have an automatic tensioner? If so, it might not be working properly and not getting enough tension on belt. Check the manuals or a specific site for your car. Perhaps there is an adjustment. Inspect the belt both sides - may also tell you something. If no luck, remove belt and check all components turn freely. Also make sure all pulleys line up properly.

I know nothing about that car, but have serpentine belt on our E320 that broke once a high speed no access elevated expressway. Not fun! Get her properly fixed! Shop did mine for about $200 plus towing paid for by insurance co.

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Graham
85 300D,72 350SL, 98 E320, Outback 2.5