My friend thinks there's a ghost at our hideout...?

Your friend is wrong, we can not say that it was a ghost just because you've found a broken glass, and the displacement of the cards, since the presence of ghosts and first of all based on things visible! Ok!-

Additional data:
I agree with @ Sarah in part, but disagree on his own, with regard to Scripture, the Bible does not function as a shield against the visitors, but you should read and ponder the scriptures from his believers, as there is word of God!
As to the alleged crosses, I'll tell you!
The demons are not afraid of simulacra, on the contrary, where there are the symbols of idolatry the demonic presences intensify even more, the same applies to the alleged blessing of the priest.
It 'important to remember that, not necessarily has to be the presence of exorcists, but faith and prayer that the believer stands directly "only" to God, put away the demons through the intercession of God's power
Since it is God who sends the demons, for the prayer requests of the faithful!