Hello sir, I'll tell how I would have approached this situation:-
1. First talk to the parents and find out why they are against the idea of their daughter dating a white boy. Find out the core reason. Is it because:
a. They're afraid of what others would say.
b. Daughters future.
c. Hindu values.
If we analyze/deconstruct further:-
1.They're afraid of what others would say.
This stems from an idea of a belonging to one particular group. Although the parents are not orthodox, they still haven't renounced their membership with the clan. They've just stayed away for sometime for personal benefits. Should they continue with their identifying with some group or not is their call, but one thing is clear, they do not have the rights now to control their daughters choice. If they themselves haven't subscribed to one way of life, don't expect your daughter to tow your line. So, check with the parents.
2. Daughters future
Genuine reason. Dating at 18 and maybe getting pregnant or ending up being a single mother is a sure recipe to screwing up ones life. No parent would want it to happen to their children. So what's the Alternative?...either teach the daughter about safe methods of having physical relations.( hope they don't, but just in case they do). Or convince the daughter to hold still till marriage.
One can't tame biology, such things are bound to happens and in a free country like America, one cannot expect to control or impose upon the other
c. Hindu values
I don't think, this is in the priority item for them. You said, they weren't traditional, so I don't think the daughter would have imbibed any values either.
They need to realize that , they may be Indian at heart with roots rooted in India, hence they can walk away for sometime and still come back but their kid is an American(assuming she was brought up there). No matter what they say and cry, she will never understand what it means to be an Indian or a Hindu unless she's been brought up that way. So, there's no point going down that road.
I suggest them to sit with their daughter and find out reasons over why she feels so, what she is seeing in her future etc. Sit and talk, introspect ans then decide...
Is dating a white boy bad??--->No
If bad--->find out why.
Should u talk to the girl---> Damn yes
Introduce to the girl about Sex, Marriage, culture, religion, ancestry--->Darnn yesss.
So ask them to have a proper dialogue with the young lady, rather than worry. Worrying only worsens the situation.
Do all of this one step at a time and then decide.
Parents will have a lot of priorities to be set here, some like
Race--Indian vs white
Culture--American vs Indian
Religion/Spirituality--Theirs vs daughter
Happiness---Theirs vs daughter.
This is just a coming of age issue. So, a proper dialogue would be enough to solve unless of course if the kid is'nt woke!!