How much does the horse weigh? (proper weight, not overweight!) Is he totally fit and healthy? Does he have good conformation?
How much does the tack weigh? Is the prospective heavyweight rider fit and experienced, or an unbalanced unskilled lump?
A good rule of thumb is that a horse can carry up to 20% of its own body weight without any problem. It will be able to carry up to 25% or even more when the rider is skilled and considerate, and the horse fit, or if only short, slow rides are taken. Bear in mind that a rider who is much larger than usual may well need the horse to be fitted with a different saddle for the comfort f both of them.
Of course the general build of the horse must be taken into account, too, and any weaknesses considered carefully.
However, most horses are more than capable of carrying more than most riders imagine.