Physical suffering -> physical strength
Scar tissue is stronger than normal skin.
Physical suffering -> physical weakness
If cut someone's achilles tendon they will never walk again.
Physical suffering -> mental strength
Even if you cut someone's achilles tendon, the forced change of lifestyle may make the person wiser.
Mental suffering -> mental strength
A person may be greive when one of their parents dies, but this experience may make them realise the value of their relationship with their remaining parent and make more of it.
Mental suffering -> mental weakness
The world is, unfortunately, littered with examples: the bullied become bullies; the marginalised become terrorists; those that once loved become bitter and twisted. Although it could be argued that some of these responses are a form of strength in that they protect the person from further damage, I consider mental strength to be the ability to make the right decisions even when they are difficult.
From the examples I have thought of, I can only conclude that this statement (at least in its literal interpretation) is wrong. Suffering can make you stronger, but it seems it can also weaken you. There is probably a threshold for every type of suffering, for every person, at every time above which they will be damaged, but below which they will be strengthened.