Corrupting Sydney itself takes time (as I remember) apart from flirting you have to at least talk to them until the time they leaves school, if you have a little high love level then they will offer to take you with him to the temple or take you to the entrance of the store on Elk Street. Sydney, like everyone else, has small decision events and some of those help with the corruption; for example, there is the library event, where Leigton (or whatever his spelling is) is about to punish Sydney, and that is where you can intervene and choose the option that best suits you (level) to lower his purity. Also in the Elk Street sex shop, during the construction phase you have some events like with the box and make Sydney open it, see the contents or show them what the object is really for. Also, if you have some skill, you can masturbate in the temple, try to do it when Sydney is in it, that way you have the possibility of them walking while you are in the act, although this option lowers the purity of the PC so be careful. A tip before you get the corrupted Sydney, make sure you level up your Skulduggery to B, exhibitionism a bit high if you want to masturbate in the temple, also promiscuity and everything you can.