The problem is, now that Rex has been bathed, can Mike give him flea treatment right away? The answer depends on your method of flea treatment, but is basically, yes. The trick is that, depending on the method of treatment, you may probably want to completely dry your dog before applying flea treatment so as not to dilute the medication when applied. Some flea treatments suggest waiting for a day to two for natural body oils to return, which may allow for better distribution of some medications and protect your dog's skin from harsh chemicals. However, when fleas are around time may be of the essence!
Your dog is not going to be very happy about having fleas, they are itchy, pesky, critters. You will probably notice your dog scratching, licking and biting at his coat. The sooner you can get rid of them the better. You may need to completely dry your dog before applying the treatment and if his skin is already irritated from fleas, a thorough drying may be even more irritating, so be prepared to be understanding and gentle with your dog.
It is pretty common to discover fleas on your dog during a bath, and then need to apply flea treatment immediately afterward. You have a few options to ensure the effectiveness of the medication. Carefully dry your dog so that medication applied to a damp or wet dog is not diluted, which can reduce its effectiveness, or wait for a day to two until your dog is dry and body oils have returned. You can also choose to use a flea treatment such as oral medication that is not affected by your dog's moisture level.
Remember fleas are pesky, but treatable, and rarely cause serious complications or disease. Clean your dog's bedding and environment as well to prevent reinfestation.