No I haven't. But I do believe RIT dye says something about this.
This is what the company that makes RIT dye says.
"For more intense color when dyeing fabrics containing cotton, rayon, ramie, or linen, add 1 cup salt to the dyebath. When dyeing nylon, silk and wool, add 1 cup white vinegar to the dyebath. If possible delay adding the salt or vinegar until 5 minutes after the fabric has been in the dye bath. The delay will help to promote level dyeing.
Add 1 Tablespoon laundry detergent to all dye baths to help promote level dyeing.
The longer the garment is immersed in the dye bath, the deeper the resulting color. Items can remain in the dye bath up to 1 hour as long as the water remains hot. You'll also have to make sure the item receives constant agitation or stirring."
So your friend seems to have a good idea. I may try this.