It's not hard to get France to stay neutral. They made some wrong assumptions when they jumped in. First, they assumed a short war. The patriots did not make the big push, though, preferring to fight not to lose once they knew they had France, Spain, and the Netherlands on their side. Don't know where that big push would come from, though. Just as the colonies were too big for an easy crushing put down of the rebellion, so too was Britain too big for a quick victory. This aspect should have been foreseen. It had been a long time since there were quick and easy wars.
Second, they assumed they would gain a massive trading partner. The Americans abandoned France at the negotiating table and were not much of a friend afterward. As the natives were to find out, promises didn't mean much to the US. Can't say if this one could have been foreseen.
France was looking to make some gains besides separating the colonies from Britain. They ended up empty handed, though.
The correct course of action should have been to fund the Patriots, but stay short of open warfare. It's debatable, but I think the Patriots would have lost if they didn't come to compromise. OTL, they didn't consider compromising once they knew the French/Spanish were jumping in with both feet (Spain didn't jump in as blindly as the French did, and came out ahead in the end). TTL, they might be a little more flexible in '79ish timeframe.
I have my doubts that the two sides could reach agreement, though. Which leads to questions about what comes next. Seeing the Patriots sitting at the bargaining table, with the likelihood that France/Spain would be left out of any gain for their money, might lead to reducing/eliminating any support, which then causes the rebellion to collapse from lack of supplies. This then emboldens Britain to retract any generous offers. The loyalists come out of the woodwork as it looks like the uprising is failing. OTL it was neighbor vs neighbor with the patriots gaining the upper hand and being brutal against the loyalists. A lot of people were on the fence, or secretly loyalist staying out of the fray. TTL, the pendulum swings toward the Loyalists. Could get ugly. Back in Britain, though, folks are getting tired of the war. The war hawks point to that victory that seems so close. A big mess, which doesn't satisfy anyone.
I think it ends up a British Victory....for now. Neither side will have gotten what it wanted, and it all depends on whether they slowly cooperate toward a workable union.
France will be far better off financially. The King will take some flack for not stepping up to the plate when there was a chance to tackle the hated British.
Spain will be without Florida, which aside from prestige, is no big deal. They also don't have the OTL claim on the southeast (they claimed it by right of conquest from the American Revolution).
Both France and Spain are still looking for an opportunity to go at it with the British. They're likely to see the war weariness of Britain as a time of weakness and look to strike a spark. Not sure where/when that spark would be. Sometime in the 1780's.