Assalamu alaykum,
Majority of these customs have been passed on from the Hindus.
Mufti Ebrahim Desai said:
To refrain from any act, marriage, journey, etc. in some months, Muharram, Safar, etc. was a custom of the pre-Islamic era based on superstition. Rasulullah 
condemned that by saying, ‘Laa Safara’ (There is no evil in the month of Safar). There is no restriction on marrying in any part of the year and neither does any part of the year itself have any negative consequences. And Allah Ta’ala Knows Best
Some people have this ideology that the month of Muharram is an unlucky or a cursed month, due to the marthydom of Hadrat Husain 
. Again, all these beliefs are baseless and wrong. Muharram is one of the four sacred Months out of twelve. The Holy Qur’an and the sunnah of the Holy Prophet have liberated us from all these superstitious beliefs of the non-believers.
[Khayr, may Allah swt enlighten our hearts with the truth and guide us on His path and keep us steadfast. Ameen]
So go ahead, and get her married whenever you want to. 
And may Allah swt put infinite barakah in the wedding and give them the best of both worlds. Ameen.