Some glues will break down slightly with a standard 'multi-surface cleaner' in a spray bottle from your local retail outlet.
It is mild enough to be fairly certain it can be used on most surfaces without damage or staining.
Simply spray it on the glue, wait 5 minutes and rub gently with a clean cloth.
It really depends on the type of glue and the surface. Tiles and vinyl should be no problem at all, but be very carefull with real wood flooring. Use a hard to see test area first.
If in doubt, simply tell the client that he must call back the floorers, who will be insured to use the correct chemicals.
Hope this helps - and good luck.