If i my pet gets lots of strength from my pet, will hearty (a pet ability that gives strength) be canceled out like my friend said
"Hearty" simply increases the maximum amount you can train your pet's Strength category. I don't know what your friend means when he says it can somehow be canceled out. If you hover your mouse over the strength column on your pet screen it will tell you what stats the strength category applies to. One of these categories is "Damage". For instance, if your pet gets a talent that boosts damage then a higher strength will be factored in to the percentage of damage increase.