As far as I know, there is no rule limiting armour saves in Mordhiem, nor is there any '1's always fail' rule. That being said, not all armour saves add together. I don't know the rules for the Scaly Skin you are referencing, but if it only says "Grants a 5+ armour save" then it won't stack with Gromril, which does not give +3, but instead grants a 3+ armour save.
So, 3+ for Gromril then down to 1+ for the shield, then down to 0+ for the horse. So at S5 (down to S4 for resilient) your armour save will be 1+. He better hope he gets some good crits.
I would recommend your friend invests in some gromril axes. S5 (down to S4 for Resilient) with armour piercing and gromril piercing will take your 0+ armour save 2+. Mighty blow will take this to 3+.
For people other than your friend I would recommend getting a handgun and superior black-powder. Your armour save would only be 1+ at range and resilient wouldn't help against the S5 so your save would be down to 3+. Of course the best weapon of all would be the warplock pistol with superior black powder and failing with a 6 on the misfire table. Good luck with that :-).
Also, magic that ignores armour saves would help, but there is not a lot of that. Word of Pain and Summon Squirrels off the top of my head.