The Japanese language has the sound /w/.
As a reason for her pronunciation, I suppose that the word Ďwomaní is used in Japanese, and pronounced /Ʊmən/.
Iím sorry I donít know any literature about the matter at present.
How about trying the procedure shown below (for her) to produce the sound /w/.
1. Pucker up the lips as forward as possible.
2. Send out a volume of air from the mouth, feeling its(that is; air) being sent out in the form of a column or a cylinder.
3. 1 and 2 should be done with the opening of the mouth as little as possible in the shape of that of an octopus.
4. The idea or feeling of sending out is important.
I hope the procedure will help. //// Not a teacher ////