Malaysian / Singaporean

Roti King in Euston - best Roti Prata. Teh Tarik, Nasi Lemak. Mee Goreng. Everything there is super good. go during off peak hour as it is super popular and small.

Rasa Sayang in Chinatown - for decent Singaporean food like CKT, Hokkien Mee, Chicken rice.

Another one they can try is : C&R in Chinatown

and Old Town - More Hongkong style. Some people like the Chicken Rice there.

Japanese -
Jidori in Covent Garden - for Japanese
Koya for Udon - fresh udon made in the shop
Lots of Ramen places around soho area - but Tonkatsu is my fave.
Misato in chinatown - my japanase hairstylist favourite jap place. cheap and hole in the wall kind.

Dumpling Shack at Spitafield Market - Good Spicy Wan Ton Tan Tan Noodle
Beijing Dumpling in Chinatown

A lot of Indian restaurant in London are northern indian. As long as you ask her to stay away from BrickLane she would be fine. My personal fave is Sri Lankan Indian restaurant - Hoppers in Soho or Oxford street
Kricket in Soho - If they want to splurge, go Gymkhana for michelin Star Indian.

Dotori in Finsbury Park.
Yori near Soho -
Assa in Soho - for Korean Hotpot
Seoul Bakery near Russell Square - hole in the wall for quick korean fix