There is a protected place for primitive tribe Lepchas in Sikkim, where even the domicile holder of Sikkim cannot buy a land.
Sikkim was an independent country, which was made an associate state as late as 1974. Next year in April, when Chogyal (king of Sikkim) was removed by India, the elected representatives of Sikkim asked India to make Sikkim a full-fledged state.
Hence, the constitution was amended by 36th amendment on April 1975 to make Sikkim a part of India. Sikkim was very sensitive towards its identity. So Article 371F was added, to protect the unique identity and old laws of Sikkim.
This article in our constitution protects our unique Identity and culture. And not allowing other people to buy property in Sikkim is a part of it.
This being the situation you may not even be a joint owner of the property proposed to buy in your friend's favor.
Better not to take any risk, instead you can give her a loan and make a loan agreement for the amount lent.