There is no particular way to mend a broken heart but if you are genuinely in love with the person, you will no what to do
as for me, i will advice you firstly not to bring up love issues with someone that just experienced heart break..the least thing she needs is someone bring up something that caused her plight in the ist place.
Most importantly be there for her without being a pest(being with her all the effing time cos she needs space to reflect and heal up)
give her unselfish advices...let her grieve cos she needs to grieve
dont go about buying her gifts but make her laugh or you can take her to palces that are fun to be at so that she will get her mind off the pains
by this time you must have
(1)gotten her to know that some good people still exist
(2)made a good impression on her
(3)opened up a soft spot in her heart and if paradventure, you wanna date this girl, you have done 90% of the wooing process!!
When you guys are alone(by then she now have smiles on her face all the time), you can now tell her your mind cos dating her(if you are single) if you love her and she loves you back is the final step in healing her broken heart and always remember that this popular saying by gurls "i want a guy who will show me that all guys are not the same".
So if she should agree to date you, make sure you erase the mistakes her former guy made with her.
Cheers!!..goodluck in your adventure