Your friend will not likely be allowed to enter Canada while he is still on probation for purposes of attending a wedding.
While exceptions are sometimes made on compassionate grounds, etc., they are pretty much reserved for funerals of close family members or for purposes of visiting a terminally ill family member, things like that.
The wedding of a brother-in-law is not likely to be deemed a sufficient basis to thaw the cold hearts of the customs and immigration people.
Even after the probation period ends, there is a lengthy waiting period before the person can apply to be allowed into the country and, in the case of sexual offences, the customs & immigration people are likely to take a very close look at the circumstances of the offence and the offender, all of which also takes time, if they will entertain the request at all before the usual waiting period.
That said, it doesn't hurt to try ~ but depending how far in the future the wedding is, your friend should probably start looking into the process asap. He can get the necessary guides, application forms and instructions here:
http://www.cic.gc.ca/english/applications/rehabil.html