Gretchen Rossi and Slade Smiley were caught in a CRAZY scene last night when a bill collector angrily hit 'em up for more than $100K in late child support Slade allegedly owes for his son ... an 11-year old who's fighting brain cancer.

The "Real Housewives of Orange County" couple had just stepped off a red carpet in Hollywood last night when the bill collector jumped out and shouted at Slade, "Pay your child support! Your son has cancer."

As TMZ reported, Slade owes a major chunk o' cash to his ex-gf Michelle Arroyo. Their son Grayson was diagnosed with brain cancer at age 6 and has had 11 surgeries.

In the video, the collector repeatedly says Slade owes $160,000 in back child support -- but Michelle tells TMZ the total was around $130,000 ... until she agreed to reduce it to $95,000 in a recent court hearing.

She claims Slade is currently paying her $775 per month. We called Slade's rep for comment ... but haven't heard back yet.