Joshua Lodge, 19, was accused of inappropriate behaviour after he was caught by Grint's father with the young girl - whose age has not been released - at the 2006 party which took place in Hertfordshire, England.
Lodge appeared in St Albans Crown Court on Monday (03Sep07), where he was due to stand trial for sexual activity with a child - charges which he has always pleaded not guilty to.
However, Lodge walked free from court with a caution and was forced to sign the national sex offenders register.
Prosecutor Ann Evans says of the decision, "I take the view that the nature of the offence was such - two teenagers at a party where, unfortunately, on all sides, too much drink was taken - that a caution would be appropriate."
In a statement released by Grint on Monday, the actor condemned his former friend and expressed his support for the victim.
He said: "I am totally appalled and disgusted by the actions of Mr Joshua Lodge. I am 100 per cent behind the victim and indeed all victims affected by this type of crime."
Source: WENN