Describing the pair's behaviour, the source added: 'They were dancing around and she was chain smoking and drinking lots of Jack Daniel's and cokes. They had a bottle.
'They were all over each other, kissing constantly and dancing. He was lifting her up and spinning her round a lot.
Letting their hair down: It is unknown when they plan to tie the knot but it appears they've become close in a short amount of time'They seemed to be really happy and caused quite a scene - lots of people were turning their heads to have a look at what was going on.'
Over the weekend the former X Factor star broke the news to fans on her Twitter page, before sharing a picture of the happy couple and their new additions - two heart shaped tattoos on their ring fingers in place of a more traditional diamond.
Boozy day: Waissel was seen swigging from a Jack Daniels bottle as she continued her party afternoon
Time out: She announced the happy news on her Twitter account after they got matching tattoos on their ring fingersThe 26-year-old singer wrote on her Twitter page: 'Just woken up with a huge smile on my face can't believe that we are engaged!! @Brad_Alphonso.'
Katie, who has been working on new music out in Los Angeles, later added: 'Making great music out here, got engaged to the man I'm going to spend the rest of my life with,and have all you to share it with!so happy!'
And model Brad, who appeared in Madonna's Girl Gone Wild video, also expressed his amazement over the engagement, tweeting: 'Holy s**t!!!!! It's for real !!! Love it!!!!'
Waissel previously dated Radio DJ Greg Burns, who she was spotted holding hands with just two months ago in May.
Matching: Katie Waissel shared a picture of their heart inkings as they revealed their plans to marry