I have the following information:
The maximum length of stay in Europe for non European Union passport
holders is determined by the Schengen accord and is currently limited
to 90 days within any 180 day period. The important thing to note is
that you may not leave the Schengen Visa area for a day and return to
restart the 90 day clock. In other words, you must be absent from the
Schengen area for at least 90 days in a 180 day period to be legal.
So if you friend has a multiple entry visa for 5 years, he can stay a maximum of approximately 5 * 180 days (Stay 90 days, be absent for 90 days, and that for 5 years).
It isn't allowed to stay longer but I heard that those rules are rarely enforced and penalized, but if you're friend looks really that dodgy it might be better to not risk it ;)