I keep getting emails from my friend, advertising malware protection and viagra....?

It's a worm. Worms purposely send effected email to everyone on the person's address book. That's what they're mostly known to do. Unless u downloaded anything from the suspicious email then ur alright (although i've read of a worm that can automatically download itself once opening the email msg). you can pretty much tell if u have a worm if it automatically starts doing crazy and weird things on ur email account. Ur smart to delete the messages. But ur friend needs to get an anti-virus software (like norton, mcafee, malwarebytes) before someone agrees to a penis enlargement! ;3