L1 visa valid if you leave the company and rejoin after a few years?

I think your friend should consult a qualified immigration attorney. When he left in 2011 did he leave the U.S.? If not, he has accrued unlawful presence. Also, why did he leave his job? I would suggest that you schedule a consultation with a qualfied immigration attorney to discuss the situation in detail and be sure the correct choices are made.

So long as the company did not withdraw the approved L-1 petition, his L-1 visa stamp would still be valid. If the company did withdraw the L-1 petition, it could file a new one and use that approval along with the unexpired L-1 visa stamp to ge thim to the U.S., but in that case it would probably make sense for him to apply for a new visa stamp as well.