If a friends who has mono spits a little when they laugh and that infected saliva came into contact with my arm, should i be worried?

Mono is transmitted via secreations/ fluids in contact with mucus membranes or blood. So if the skin was intact ( no scrapes /cuts) there should be very very little concern ( we never say never in medicine) .Would though wipe it off immediately as if you touch it and then inadvertently put your fingers in the mouth there is a risk of trasnmission. https://www.cdc.gov/epstein-barr/about-ebv.html