Growth requires energy.
If his calories in do not meet his calories out then he can not grow even if hes taking 15 grams a week of AAS.
Its simply the laws of nature.
That said his body will use his fat stores to form muscle if he doesn't eat enough.
Lets estimate hes 10% bodyfat, he has 15 pounds of bodyfat, not all of which is usable.
So probably 7 pounds, thats 3500 * 7 or about 25k calories.
Thats a few pounds of muscle sure, but since he will be losing the fat at the same time his weight will stay about the same.
Did he grow, yes, will it slow him down from bulking up, no.
This is purely hypothetically, using a normal cycle, or even a peasly one like hes speaking of his body isnt going to be able to do much to convert that fat into muscle.
He will get a little stronger for a while then revert back once hes off cycle.