Kimberley Walsh says she was "sad" that Sarah Harding couldn't make her milestone 30th birthday because of her bandmate's stint in rehab.
Cheryl Cole was not at the bash either, as she was unwell, while Nadine Coyle was in the US.
Kimberley, who was presented with a diamond ring from her bandmates, told OK! magazine that she and Sarah normally celebrate their birthdays together.
She said: "She (Sarah) texted me in the morning. In the past we've had joint parties and we were both 30 this year, so it was sad not having her at my milestone birthday."
But she added of Sarah's decision to go to rehab: "It is brave as some people can't face up to things, and she obviously knew she had a problem and could help herself and I'm proud of her for doing that.
She said of Sarah: "We haven't celebrated her 30th yet, so it's just an excuse to have a night out.
"We'll definitely go out for dinner with Cheryl there too."
Sarah recently returned from rehab in the US, where she was treated for alcohol addiction and depression.