My cousin, who resides in China is planning to come to Thailand on a 2-month Tourist Visa.

As me and her are going to Malaysia by land after a few weeks once she's here, she does not need a return ticket from Thailand to China. However, she purchased a ticket for Malaysia to China as her return plan. The embassy told her it is allowed to obtain a tourist visa if you have a return ticket from a neighbor country, so getting the visa this way is feasible.

My only concern is at the airport here, will they ask her a return ticket from Thailand to China? In that case she will show them her return ticket from Malaysia. Will she encounter any problems?

Thanks.

With a proper visa, like he has, no ticket out of Thailand is required.

I don't think you need a return ticket. I've come to Thailand on a simple tourist visa and never was asked for a return ticket. Though, I generally make one up on my laptop and print it out if Immigration does ask.