My wife and I went in 2007. The bears were real pests. Several guided hikes were closed because of an aggressive grizzly. The recommendation was strong to not venture off the beaten path because of this. That all said we had an awesome time. Know how to deal with grizzlies. Don't make eye contact with them. They don't like that. If you do back country trails, camp in a group. This might be required. Invest in good camping gear. You will be miserable with cheap shit.
Just be aware that bells and pepper spray are bear deterrents, not bear repellants. There is an old joke in the mountains . . . You can be mindful of the bear danger by paying attention to their scat. Young bear scat tends to have a lot of huckleberries and lily bulbs. Scat from adult bears tend to have little bells and smells like red pepper. I should have mentioned this. My sea-level lungs often give out before my damaged knees at 5,000 feet. Some Glacier trails top out over 7500 feet. Pilots go on oxygen at 10,000 feet! Aren't we all. July and August in the Rockies is often clear and nice, but it is also when big thunderstorms build up in the afternoons and you have to be very aware of lightning.
Red is probably right about stuff like pepper spray being ineffective against bears. Since you probably don't want to hike with a high powered rifle strapped to your back I would recommend a .44 magnum revolver. Plenty of stopping power and the noise should deter any other bears in the area once you shoot the attacking bear.
While it is true that firearms are no longer prohibited in national parks, you still have to follow state and local gun regulations. Montana's gun law is fairly liberal, they will recognize concealed carry permits from 40 states. Carrying a gun in a pack is a concealed weapon and you must have a carry permit. Wearing a gun in a holster is technically legal but it annoys and frightens most of the other people on the trails. Parks in other states can be much more restrictive. Federal laws still do not allow guns in federal buildings, such as park visitor centers and other park buildings including private vendors. They also forbid hunting, target shooting or even firing a gun in the park. No one will object to a person using a gun to save his life from a bear attack. But if you shoot a bear under any other conditions, including simply being frightened because a bear is near, you can be subject to serious federal laws and be jailed for it.
If he carrys the weapon in his backpack nobody will know unless he has to shoot a bear. I would rather risk the legal consequences than risk being torn to shreds by a crazed grizzly bear. This assumes that he is smart enough not to go shooting at any bear he sees off in the distance
Since this park is close to the Canadian border I'll be flying there so I won't be bringing a gun in my carry on luggage. While Red is right, the bear spray is in a bigger canister than the pepper spray women buy to deter human attackers and shoots 15 to 20 feet. I'm sure some of you know this, but I didn't want to give anyone the idea I was going with a chap stick size tube up to Montana and fighting bears with it. Red has unknowingly given me something to brag about though by saying that pilots go on oxygen above 10,000 feet. Last year I went to Rocky Mountain NP and snowshoed up to an alpine lake that is slightly above 10,000 feet. I would feel like more of a man's man if there weren't some children and post-menopausal women up there too! But seriously, I think the altitude will be more of a factor this time because last year I was in Denver for a few days and probably got a little acclimated. This time I'm going from below sea level to the top of the continental divide by dinnertime.
