Male Dog Problem. Is This Normal?

My friend has a 1 year old male jack russell. I usually bring my two female cairn terriers with me when I visit and they all get on great. One of my cairn terriers is 3 years old and well able to handle herself and he tends to leave her alone as she puts him in his place. My other cairn terrier is 5 months old and he doesn't bother with her as she has never gone into heat so they all play together however my other friend has a ten month old female pom crossed with a terrier and when she visits he never leaves the poor dog alone .He is constantly tries to mate with her even though she isn't in season, he sometimes gets very aggressive and this only started to happen after she went into her first heat. She is a very gentle dog and never barks at him or anything so he takes advantage. My friend doesn't bring her dog anymore because of this. Is it normal for a male dog to constantly want to mate even if the ***** isn't in heat. In my opinion he is overly aggressive with it. I have never had a male dog so I don't know if this is normal. Just so you know I have never brought my dogs over when they have been in heat and never will and my friend doesn't either. Any help from male dog owners would be very much appreciated. I have suggested to my friend to get her male neutered as not only is he aggressive with mating but he is very high in energy and sometimes seems frantic with frustration.