Hi Uyap,
The only method to be 100% certain that one will not end up being re-born in one of the lower realms is to become a sotapanna. This is true for Buddhists and non-buddhists a like.
When practicing the metta bavana, if one is to wish for all beings to attain nibbana, the only way that can happen is if they understand the dhamma. So we can even wish saying “may all beings understand this Dhamma, eliminate the chance of being born in the lower realms and be free from all suffering by attaining nibbana.”
Although this may sound selfish, each person has to understand the Dhamma for himself. No amount of metta bavana will lead to someone else to understand the dhamma. Also it is foolish for us to think that we can get the infinite number of beings to attain nibbana (by say teaching them the Dhamma or in any other way). If this could have been done, the Buddha would have done it. So the only thing we can do is hope / wish that everyone will be able to understand this dhamma and attain nibbana. This emitting of metta can help them and that’s all we could do.
Interestingly the Budha didn’t categorise people as Buddhists and non-buddhists. The categorisation was pruthakjana and arya shrawaka i.e. people who have not attained at least the sotapanna anugami stage and others who have.