If your MIL is bringing this up to you guys then there is a major problem and she is reaching out to help your FIL. However, I would not recommend that you step in as the others noted it could cause more problems but perhaps offer her tools to discuss his need to break from the evils of porn.
All porn is bad for the human soul and it corrupts our collective humanity. It is deadly to the human relationship to God and to each other as it distorts the beauty of the human person. Your FIL probably does see it as wrong, knows it is hurting his relationship with his wife & God (if he is a man of faith) but has been seduced by it’s illusory promises. He may not even get sexually aroused by it anymore but be unable to stop viewing. He most likely made excuses about it for a long time because his soul knows that it is sin. He may even have made promises to himself or MIL about stopping it, but after awhile finds himself going back to it. Yet, he cannot break this on his own, he will need Christ as his center, and help from various points (i.e. firewall, computer in public space, meetings for those addicted, marital counseling, frequent reception of Sacraments, prayer etc). All good men have their sins, your FIL’s is this. He like the rest of us needs Christ and those around us to help us overcome and perserve.
However, again it is your MIL who must approach him because she obviously recongnizes this is a problem for him and their marriage. Or else she wouldn’t be talking to you about it.
Only approach him if you know that your children have been exposed to the material or may have possibly been (i.e. he was viewing the computer with them in the room but they did not actually see it). Yet, without direct confirmation you are really not able to know.
If you want to pass on information to your MIL, again since she brought it up as a problem she might not know where to turn, these might be an idea:
saa-recovery.org/ :Sex Addiction recovery group
sanon.org/ :for spouses or loved ones affected by a sex addiction
familylifecenter.net/strugglewithporn.asp : this was recommended by Steve Woods from
dads.org/ & you can find other information on the
dads.org site (both sites are Catholic)
Remember to also pray for him & your MIL. They will need it as they battle this and heal their marriage

I will too!