"Real Housewives of Orange County" rivals Tamra Judge and Gretchen Rossi never saw eye-to-eye on the show, and their feud only seems to have strengthened with time -- because we've learned Tamra has donated money to help honor Gretchen's late stepson.

Tamra put $500 towards the GoFundMe for 22-year-old Grayson's memorial and medical expenses ... inching the campaign closer to the halfway mark of $70k.

"Real Housewives of Beverly Hills" alum Teddi Mellencamp also put $500 in the pot -- and more than 400 people have donated so far.

As we reported, the campaign was set up by the family of Michelle Arroyo -- Grayson's biological mother, and ex of Gretchen's fiancé Slade Smiley -- after her son passed this month from brain cancer.

There's no love lost between Michelle and Slade and Gretchen, which might have played a factor in Tamra's decision to financially back Michelle.

The donations will help with the cost of a memorial to honor Grayson, as well as outstanding medical expenses -- of which Slade is responsible for half, but allegedly never paid ... according to Michelle.

She told us Gretchen pretended to have a relationship with Grayson to gain sympathy online, and claimed Slade hasn't made child support payments since 2019. Sources close to the couple told us Michelle's accusations are absurd and need to stop.

Both Tamra and Teddi encouraged listeners on their iHeart podcast "Two T's in a Pod' to donate to Michelle's GoFundMe. Tamra pointed out how Gretchen just got a nice chunk of money for going on Ultimate Girl's Trip while Michelle is having to go through a GoFundMe to raise money for Grayson's memorial.

In any event, Tamra's showing support for Grayson's mom. That ol' saying comes to mind -- the enemy of my enemy is my friend.