He should throw the muffs in the garbage then.
I have Peltor muffs that fit like clamps or a vise. Any bark would be extremely muffled. The problem is, wearing them before a bark or noise - you need to be able to anticipate the noise. Decent muffs aren't good to wear too frequently.
If his dog is barking frequently, then yeah, very likely to spike reactive tinnitus. But, I think concerts bring more dangerous consequences including more hearing loss, spikes, worsened tinnitus - there's posts here about that.
He could also look into a muzzle for the dog - one that allows drinking water - but, what might prevent barking. Take it off after a while and if the dog barks, "NO!" and put it on again. If the dog reacts to things without barking - then lots of praise and treats.
I know barks can really bother the ears and lead to spikes but I still think attending loud concerts and sports events - as these are usually at least 2 hours in duration - are more risky.