It is prohibited to enter his friend’s plow-field because you will trample what he plowed and ruin it

It is prohibited to stand over his friend’s field to look it over while it if standing at full height [ready for harvest], so that ayin hara* does not damage it. All the more so one should not look into [the affairs of] a friend in some matter that one can fear that ayin hara will harm him.

Even in his dealings and actions about which there is no fear of damage through ayin hara, if he does it in his house and his property, it is prohibited to watch without his knowledge, because maybe he doesn’t want others to know of his actions and dealings. [I.e. you are obligated to grant others privacy.]

It is proper conduct that when someone sees his friend busy at his work to bless him and say, “May you succeed in your doings!”

* Ayin hara, litetally “evil eye”, is a term used in the mishnah for jealousy (e.g. Avos 5:19). Since Hashem generally punishes through the means by which the person sinned, a logical punishment for arousing jealousy in others through unnecessary conspicuous consumption is to lose the item in question.

Therefore, even if someone is only somewhat demonstrative about his wealth, and thus you would not grow jealous without taking some effort on your own to look over his success, he is better off if you do not do things to grow jealous of it. This way he will not be punished for his share in causing you that pain.