Hi, I'd like to help you with your question.
Your husband does not have to continue to help his ex wife, especially if there are no children involved. This is a choice he makes, maybe out of guilt (if he was the one to end the relationship) or because he is putting himself and her first.
If he is helping his ex and doing this when you need him, then he is putting himself and her first. Many men have trouble after divorcing and remarrying seeing their new wives as first. They cannot separate the first relationship and the second one. They always see the first wife as the primary one and this causes many conflicts because once the first relationship ends, there should be boundaries in place and he may have trouble setting them.
Your husband could also be feeling guilty. If he ended the relationship, then he feels it was his fault for leaving his ex and therefore he should be there for her in case she needs him. Men are very in tune with being providers and most of them are responsible enough that they cannot refuse to "provide" their ex wives with help.
If there are children involved, then he will feel even stronger about providing for their mother. It is an indirect way of caring for the kids. If their mother is ok and cared for, then the kids are too.
If he does have kids from his first marriage, then those relationships should be primary until the kids are grown. That does not mean you should not be included. You should. But the kids are not at fault for the divorce so they need to be important.
But as for the ex wife, she does not need to be primary. Boundaries need to be set between your husband and his ex. They are no longer married so that type of relationship needs to end. She needs to find alternatives for the help she needs. If she is desperate and your husband is the best option, then you should be included when he goes to help. And he should check with you first before he accepts any request for help from her.
Here are some resources that can help you further:
Second Wives: The Pitfalls and Rewards of Marrying Widowers and Divorced Men by Susan Shapiro Barash
Ex-Wives and Ex-Lives: Survival Guide for the Next Wife by Paula J Egner
Second Wife, Second Best?: Managing Your Marriage As a Second Wife by Glynnis Walker
You can find these books on Amazon.com or your local library may have them for you.
If you find that you both cannot work this issue out, you may want to consider counseling to help you. Ask your doctor for a referral. If you attend church, you can talk with your pastor. Or you can search on line at http://therapists.psychologytoday.com/rms/.
I hope this has helped you,