I am from Lebanon, you could visit Israel using a Canadian or a Lebanese passort, Israel will let you in. Many priests and nuns visit Israel each year. In fact, our Lebanese Maronite Patriarch Cardinal Mar Bechara Boutros al‑Rahi will go to Israel to welcome the pope. But there's a catch.
You cannot come back to Lebanon, well technically you could, but you will be called a spy and executed :) But hey many Lebanese are now living in Israel.
In fact there's a Lebanese basketball player, he was born in Australia he played in Israel and now he's playing in Lebanon, he didn't know about the Lebanese-Israeli relationship.
The issue of travel between the two countries presents a minefield of diplomatic restrictions. Tourists who arrive to the Lebanese borders with a passport bearing an Israeli passport are not allowed entry into the country and Lebanese citizens who move to Israel can be prosecuted for high treason. Lebanese citizens are not permitted to visit Israel, nor are Israelis allowed to cross the border to the north.
Exceptions to the web of restrictions are for Maronite clergy, who are permitted to travel as part of their function within the Church.
You cannot enter Lebanon with a passport bearing an Israeli stamp even expired.
You cannot enter Lebanon with a passport showing that you entered Israel from Jordan or Egypt or showing that you left Israel to Leabnon via Jordan or Egypt. You have to have two passports.
If you visit Israel using your Canadian passport, you'll be fine, many Lebanese living in Canada have done that. But if you come back to Lebanon don't brag about it, be quiet :)
About your friend, yes he won't be allowed to come to Lebanon, unless he's a member of the Jews Rabbis and he has a permission from Hezbollah :) There were a lot of Jews in Lebanon before 1975, now you still can find few, but they hide their identity.
Being charged for treason only applies if Blackbird57 actually is one (or at least, if Lebanon considers him one)?
This is hard to answer, but from my own experience, anyone who has been in Israel, anyone who contacted Israelis by any means, is a candidate for being charged for high treason (or spying or whatever). Anyone who raise suspicions might have troubles. By law you are absolutely forbidden to make any sort of contact with Israel.
But let me tell you something, if you are a celebrity, if you are a politician, if you have power and money, you may be untouchable, we know many politicians who are now free though they have worked with the Israelis :)
If he doesn't have dual citizenship then it would also be useful to know whether it's an offense in Lebanon for a Canadian citizen who has entered Israel to later enter Lebanon.
Yes, Lebanon's visa requirements forbid entry to anyone with an Israeli stamp on their passport. But he can ask the Israeli not to stamp his passport as it was mentioned in the comments below my answer.
However if you are a celebrity, well it's a different story :) Many celebrities do concerts in Israel before coming to Lebanon and they are welcomed. The only celebrity that was not welcomed as far as I know is Lara Fabien because she sang for Israel on their national day or something and she showed great support for the Israeli nation.
Many American professional basketball players have played in Israel before playing in Lebanon, we even gave the Lebanese nationality to one of them so he can play in our national team. That decision was not welcomed at all, but well... politics :)