Vtyjkyuk

Asking for a friend.

He does not hold any dual citizenship and only has a Lebanese passport. I am well aware of the diplomatic tensions these two countries hold but I was under the impression Israel did not have any restrictions on who could visit. However, I've seen contradictory answers online.

I suspect he might be questioned upon arrival. To makes things simpler, he's been living in Paris for years now and has no ties to anyone involved in politics in Lebanon.

Can he visit Israel with only a Lebanese passport? Also, even if he asks the Israel customs not to stamp his passport, can Lebanon find out he's been in Israel? Can he get in trouble when he decides to visit Lebanon?

He can apply for a visa.

Entering Israel, like any other state, requires permission in the form of a valid visa, unless it's for specific reasons by citizens of specific countries. As an enemy state, Lebanon is obviously not one of the countries exempt from a visa.

But, it's Israel's policy to allow everyone, including citizens of enemy states to apply for a visa. He should contact the Israeli representatives in France to start the process.

Again, he can start the visa process. It's not guaranteed that he will be approved for one.

Finally, regarding Lebanese authorities finding out, Israel doesn't stamp passports but gives slips of paper, so there is no problem there, but he must make sure to not enter from the land crossing from Egypt or Jordan as the Egyptian/Jordanian authorities will stamp the passport with a stamp from a border crossing with Israel. Also, usually visas are put in the passport; obviously an Israeli visa in the passport is a dead give away, so he would need to ask during the visa process if it's possible to not attach to the passport.

And, depending on his notoriety he would need to look out for public signs that he has been to Israel. For example, posting a picture from Tel Aviv as your FB profile picture is probably not a good idea.

According to this table in the Israeli Ministry of Foreign Affairs (MFA) site, Lebanese citizens can apply for a visa, but it "requires MFA confirmation" (a comment in common to many Muslim states, as well as North Korea).

I can't tell what requirements does the MFA have for this confirmation, and it's probably not public information. I would guess it involves providing a good reason and/or strong evidence of lack of negative intentions.

This thread discusses the similar case of an Iraqi citizen. According to one answer, These countries are in a different and restrictive category, and the best advice is that entry will not be easy and will require special circumstances and justification.

As other answers indicate, your friend can apply for a visa. However, s/he risks the possibility of interrogation by the Israeli security services upon entry - as we have recently seen in the media even for people from the US. If your friend is active in politics or otherwise related to significant people in the public sphere, this chance increases.

Also there might be similar repercussions on the Lebanese side, I'm not sure.

I don't suggest that your friend not try this, but I do suggest caution and possibly "lawyering up" to some extent if s/he gets the visa - in case your friend is detained on entry and not released (which is perhaps not very likely but possible).