There is no standard solution to avoid vaginal farting. But you could try the following:
1.      If you do it ‘doggy style’ the girl can try to rise or lower her upper body, making the penis enter from a different angle.
2.      Try short instead of long deep thrusts. When the penis stays in the vagina deeper, air might not be pumped in to begin with.
3.      Change position.
4.      Use a water-based lubricant when you start, applying it to the penis and/or the vaginal opening.
5.      The girl can spread her legs more or keep them closer together.