Right guys i need your help for a friend!!

So a friend of mine is worried his girlfriend isnt pregnant to him as they slept toghter the first time a few weeks later found out she is pregnant,anyway i need you guys to help me convince him that this babys his by the dates of her last period and so on. she had her last normal 3 days period on the 17th of decemeber it finished fully on the 20th anyway she had sex with her now xbf that night she had come off, this was the last time she slept with him , a few days later she had slept with my friend on the 25th onwards later on in the weeks she found out shes pregnant they go for a dating scan and gets told shes 13weeks and a half gone, up till today she is 15 and 5 days gone as you can see in the picture would you say this baby she is carrying is my friends or is there a chance the babys ger xbf. Iv worked it out and it all counts down to my friend been the dad. What do you all think?? He needs some advice hes going out of his mind bless him. Also i highly doubt she would fall pregnant a day after her period and it would also work out she would be further if what she had was brake throw period.. what do you guys think??