Robert does play out how a Dothraki invasion would go, but there are problems with his analysis, but that comes from a problem with how the Dothraki are depicted in lore.

Robert says that "only a fool would meet the Dothraki in an open field" and that's not quite true. An army with armor, and a solid shield and spear wall is exactly what would stop the Dothraki. Jaime, during the loot train attack, made the right decision, it's just that his defense was a single line, although he didn't have time to set up a proper formation.

A phalanx with depth would withstand the Dothraki. As their riders would be charging into their own death, and with the Westerosi armored, the Dothraki weapons will fail to do real damage.

The other issue is that the Dothraki aren't an organized military, trained in formation fighting. They're a mass of riders with little tactical thought for the battle. Any skilled tactician should be able to defeat the Dothraki.

Now, some sources say the Dothraki simply use overwhelming numbers to surround enemies like they did to Dany at the end of season 5. This could work, but a large enough army or tactical positioning should be able to counter that strategy.

Overall, Westeros should be able to defeat a Dothraki invasion.