First, my condolences.
Second, if it wasn't with a gun it would have been with Tylenol, bridge, building, whatever. Firearms are tools, they can be a gift or they can be a nightmare depending on their use. Suicides are troubled people, if they really want to go, they will. If there was not a firearm to use would he still be alive? Possibly, but for how much longer. There is still a very high chance no one would get any new information to decipher what he was going to do regardless.
Safes, trigger locks, protective measures are the best one can get to protecting their firearms from being used in nasty situations. In all honesty, I wouldn't try to push the firearms are still good point. Just go about your business and when it is brought up deal with it then from a logical perspective that suicides may or may not try to find help, if they don't and can't find one tool, they will use another.
When someone commits suicide, it leaves guilt and bad emotions on the surrounding people. They will look for things to blame, including themselves, because of the persons selfish action (Sorry if this offends you, but, it is selfish to not take into consideration family and friends; especially if that person did not reach out to them). It will be rough to try and force the issue and I urge you not to push anything. If you mention you are going shooting and she starts with the firearms are bad, be careful what you say but speak your points.