As long as the grass has no pesticides or other chemicals on the dog eating grass will be safe.
Dogs eat grass as a way to settle and upset stomach since when a dog eats grass the grass is supposed to make the dog throw up.
When the dog throws up as a result of eating the grass their stomach feels better since the bad stuff in their stomach is no longer in their.
If a dog is eating grass then you should continue to let them eat the grass because they're healing themselves by doing so.