I just dealt with the same problem (twice)...here is what I found:
It seemed to be a combination of problems, for lack of a better term "gunk" and machining burrs seemed to combine to hold the firing pin forward just enough to induced a slam fire. I pulled the firing pin and polished it and the hole in the breech to close to a mirror finish with jewelers rouge and 2000 grit paper.
Once reassembled it went fine for about three weeks and then did it again.
The second go around seemed to have something to do with the spring in the Timney trigger ass'y I installed. I used the original Remington spring and under the right condition the bolt going forward would set the trigger off. I put in a higher quality spring that increased trigger pull a bit but is a much safer option it seems.
I am still trying to rebuild confidence with this gun but it probably won't last much longer in my "stable" at this rate. The simplicity of an over and under can't be overlooked...that is my next direction I think.
Really repeating here but, disassemble that bolt. Thouroughly clean firing pin and hole area. Replace spring with new. Clean trigger assembly thouroughly. Then one more time on trigger. Good oil on contact areas in trigger.
Make sure primers are not protruding outward excessively in hull.
And for gosh sakes, if he is using WD40 on the innards, set up a swat line for him to walk.
I started shooting a Remington 1100 in 1969, Slam fire . Need to have the gun checked out by a qualified gunsmith, Safety First.
I have serviced 100's, Lack of maintence Safety First.
Obviously your firing pin is not retracting.
I am sure most long term shooters have a lot of experience with this gun. Both trap & skeet shooters. This gun needs to be properly cleaned & serviced.