Can the devil tell the truth?
Even though he is the father of lies (John 8:44), the devil can and does tell the truth—whenever it serves his purposes.
The most deceptive lies often contain truths that are distorted, which makes them half-truths. When the devil tempted Jesus to throw himself off the top of the Temple, he said God would not let Jesus come to harm because angels had been given charge over him. He quoted Psalm 91:12 which says, "On their hands they will bear you up, lest you strike your foot against a stone.”
What the devil said was true, but he left out a crucial point Jesus included in his reply: You shall not test the Lord (cf. Deut. 6:16). While God's angels protected Jesus from harms that came upon him and ministered to him in times of distress (Matt. 4:11), it would have been false to conclude God would protect those who intentionally place themselves in peril with the presumption God would save them.
So yes, the devil can tell both truths and lies, but we shouldn't ever trust his words. Instead, we should trust God who does not lie (Num. 23:19) but simply is "the God of truth" (Psalm 31:5).