Is there anything wrong with having a friend (who cannot commit to be a co-founder right now) be a startup advisor to help the effort along? This is a friend who you'd prefer to be a co-founder with you but because he/she cannot commit full-time and you still would be much better off with some assistance...
@Eddy. There isn't any reason they shouldn't be an advisor but you do need to sort out some ground rules in order to keep the relationship good.
You can either formally write out your version of the above and sign it, or just print it out and both read it and agree.