Was talking to a Thai today (she speaks perfect English) we were talking in Thai.

She used the expression ฆ่าเวลา, it made perfect sense in the context in which it was used.

Is this a normal Thai phrase or was she using some Thinglish expresssion?

The context it was used, she was waiting for her husband so she went shopping to kill some time.

Reason for asking, I have never heard this used.

There are lots of hits on Google for it.
There is a thread on Pantip about it below where a Thai asked for the English equivalent for ฆ่าเวลา.
The last poster says the phrase is actually borrowed from English, the same as the phrase จับรถ (to catch a bus/cab etc).
http://pantip.com/topic/30601335

Here's an example from Thairath: ...เย็นวันนั้น อนุศนิยาเลิกงานแต่ไม่อยากกลับบ้านไปเผชิญหน้ากับศตวรรษ จึงทำนั่นนี่ฆ่าเวลาจนเย็นย่ำ...

http://www.thairath.co.th/ent/novel/Plengchaikonne/ch6

Edited May 17, 201610 yr by katana