Look at it this way, when a person with oral thrush kisses someone, you can’t “give” a Candida infestation to them necessarily, even though, through kissing some of the fungi is understandably transferred to their mouth.
However, all of us, even individuals without a Candida infestation, have a certain amount of Candida in their body, so if the level of yeast in the person’s mouth is higher than normal then it’s logical that the increase in fungi could possibly make all the difference that is needed to develop thrush.
It’s logical to assume that the same can be true with oral sex.
The precautions you mentioned should work well, with one exception; instead of using a mouthwash which would have no effect on the Candida, have your friend to use extra virgin olive oil to ‘swish’ around in his mouth for about ten minutes. This would have a much better chance of killing any Candida that may have been transferred.