Hello,
if its a problem that has just started, it could be an infection, maybe she should go to the doctor. You can say.."i've noticed a change in odour, just thinking maybe there is a change that needs to be seen by a doctor".
Or, hygene. When are you going down on her? Take a shower or bath together first to freshen up (both of you. Men can stink too!!!!) and then see...If it's clean and still smells it's her fluids..That shouldn't smell horrible. We all have a smell but shouldn't be bad.
No douche..Can cause yeast infections...Stay away!
And if that doesn't all work...Buy flavoured lotions at a sex shop..Use it for flavour and smell.
Good luck!