Onions

Question

If you buy a vegetable salad in a non-kosher restaurant that has onions, can they be eaten? a friend said no, since when the knife cuts the onion (a davar charif) the onion absorbs the non-kosher taste on the knife, and we assume that the knife had at one time at least, been used to cut hot foods. Is this true, and if not, why not? Thank you.